1. Field of the Invention
This invention relates to an improved process for catalytically hydrocracking residuums in a solvent and further relates to demetalation, desulfurization, and decarbonization thereof. It especially relates to using a dual-bed catalyst system having a large-pore catalyst as the first bed and a small-pore catalyst as the second bed. It specifically relates to recycling a process-generated distillate (boiling at about 400.degree.-700.degree. F.) as the solvent.
2. Description of the Prior Art
Residual petroleum oil fractions produced by atmospheric or vacuum distillation of crude petroleum are characterized by relatively high metals, sulfur, and/or CCR content. This comes about because practically all of the metals present in the original crude remain in the residual fraction, attached to polycyclic and highly aromatic compounds, and a disproportionate amount of sulfur in the original crude oil also remains in that fraction. Principal metal contaminants are nickel and vanadium, with iron and small amounts of copper also sometimes present. Additionally, trace amounts of zinc and sodium are found in some feedstocks. The high metals content of the residual fractions generally preclude their effective use as charge stocks for subsequent catalytic processing such as catalytic cracking and hydrocracking. This is so because the bulk of the metal contaminants are contained in pentane-insoluble, high-boiling asphaltenes that are sheet-like structured materials in the residuum petroleum oil fractions.
The asphaltenes are readily adsorbed on the surface of the catalysts, and the metals, such as vanadium and nickel, which are primarily associated with the asphaltenes, are deposited on the catalyst particles, thus blocking the catalyst pores and preventing other molecules from comin into contact with the active catalyst sites. Asphaltenes are responsible for the rapid catalyst deactivation and formation of inordinate amounts of coke, dry gas, and hydrogen that are usually observed in residual oil hydrodesulfurization. Furthermore, at high temperatures, the asphaltene molecules polymerize and cause plugging of the catalyst bed in high conversion operations.
In order to avoid this rapid catalyst deactivation due to the asphaltenes, it is a practice in the prior art to only treat a portion of the resid feed. The vaccum gas oil is separated by distillation, hydrotreated, and then blended back with the residual oil material. This prevents the contacting of the catalyst with asphaltenes. A major problem arises under the foregoing method when the production of a low sulfur fuel is desired, since the sulfur contained in the asphaltene molecules represents a significant portion of the total sulfur and has not been removed.
It is current practice to upgrade certain residual fractions by a pyrolitic operation known as coking. In this operation, the residuum is destructively distilled to produce distillates of low metals content and leave behind a solid coke fraction that contains most of the metals. Coking is typically carried out in a reactor or drum operated at about 800.degree. to 1100.degree. F. temperature and a pressure of one to ten atmospheres. The economic value of the coke by-product is determined by its quality, especially its sulfur and metals content. Excessively high levels of these contaminants make the coke useful only as low-valued fuel. In contrast, cokes of low metals content, for example up to about 100 p.p.m. (parts-per-million by weight) of nickel and vanadium, and containing less than about 2 weight percent sulfur, may be used in high-valued mettallurgical, electrical, and mechanical applications.
Carbon residue may be determined by the Conradson Carbon Residue test. This test is important because Conradson carbon precursors generate surface coke on a catalyst, and the excess formation of coke upsets the heat balance of the catalytic cracking process. In general, higher-boiling range fractions contain more Conradson carbon or coke precursors. Light distillate oils may have a carbon residue less than 0.05 percent, but a vacuum residual oil may have a Conradson carbon value of 10 percent to 30 percent. Such a high Conradson carbon content, particularly when combined with excessive metals content, essentially renders ineffective most conventional catalysts and catalytic treating processes.
The effect of such high carbon residue is that many residual petroleum feedstocks are unsuitable for use as FCC feedstocks, even if metals content and sulfur content are at acceptably low values.
Certain residual fractions are currently subjected to visbreaking, which is a heat treatment of milder conditions than used in coking, in order to reduce their viscosity and make them more suitable as fuels. Again, excessive sulfur content sometimes limits the value of the product.
Residual fractions are sometimes used directly as fuels. For this use, a high sulfur content in many cases is unacceptable for ecological reasons.
At present, catalytic cracking is generally done by utilizing hydrocarbon chargestocks lighter than residual fractions which generally have an API gravity less than 20. Typical cracking chargestocks are coker and/or crude unit gas oils, vacuum tower overhead, etc., the feedstock having an API gravity from about 15 to about 45. Since these cracking chargestocks are distillates, they do not contain significant proportions of the large molecules in which the metals are concentrated. Such cracking is commonly carried out in a reactor operated at a temperature of about 800.degree. to 1500.degree. F., a pressure of about 1 to 5 atmospheres, and a space velocity of about 1 to 1000 LHSV.
Although mostly demetalated, these feedstocks are high in sulfur. The most practical commercial means of desulfurizing such feedstocks as well as the resids themselves is the catalytic dehydrogenation of sulfur-containing molecules and petroleum hydrocarbon feeds in order to effect the removal, as hydrogen sulfide, of the sulfur-containing molecules therein. These processes generally require relatively high hydrogen pressures, generally ranging from about 700 to 3000 psig, and elevated temperatures generally ranging from 650.degree. to 800.degree. F., depending upon the feedstock employed and the degree of desulfurization required.
Such catalytic processes are generally quite efficient for the desulfurization of distallate-type feedstocks but become of increasing complexity and expense and decreasing efficiency as increasingly heavier feedstocks, such as whole or topped crudes and residua, are employed. This is particularly true with regard to asphaltene-containing feedstocks, including residuum feedstocks, since such feedstocks are often contaminated with heavy metals, such as nickel, vanadium and iron, as well as with the asphaltenes themselves, which tend to deposit on the catalyst and deactivate same. Furthermore, a large portion of the sulfur content in these feeds is generally contained in the higher molecular weight molecules, which can only be broken down under the more severe operating conditions, and which generally cannot diffuse through the catalyst pores.
In any case, the residual fractions of typical crudes will require treatment to reduce the metals content. As almost all of the metals are combined with the residual fraction of a crude stock, it is clear that at least about 80% of the metals and preferably at least 90% needs to be removed to produce fractions suitable for cracking chargestocks.
Metals and sulfur contaminants present similar problems with regard to hydrocracking operations which are typically carried out on chargestocks even lighter than those charged to a cracking unit. Hydrocracking catalyst is so sensitive to metals poisoning that a preliminary or first stage is often utilized for trace metals removal. Typical hydrocracking reactor conditions consist of a temperature of 400.degree. to 1000.degree. F. and a pressure of 100 to 3500 psig.
It is evident that there is considerable need for an efficient method to reduce the metals and/or sulfur content and/or residual carbon content of petroleum oils, and particularly of residual fractions of these oils. While the technology to accomplish this for distillate fractions has been advanced considerably, attempts to apply this technology to residual fractions generally fail because of very rapid deactivation of the catalyst, presumably by metals and coke deposition on the catalyst.
U.S. Pat. No. 3,696,027, issued Oct. 3, 1972, and U.S. Pat. No. 3,663,434, issued May 16, 1972, describe a process for hydrodesulfurization of a metals-contaminated heavy oil which comprises: (a) passing a heavy oil, at elevated temperature and pressure and in the presence of hydrogen, through a fixed bed of macro-porous catalyst particles having high metals capacity and a low desulfurization activity, (b) passing effluent from the macro-porous catalyst bed, at elevated temperature and pressure and in the presence of hydrogen, through a fixed bed of moderately active desulfurization catalyst particles, and (c) passing effluent from the bed of moderately active desulfurization catalyst particles, at elevated temperature and pressure and in the presence of hydrogen, through a fixed bed of highly active desulfurization catalyst particles.
It was ascertained that a high active hydrodesulfurization catalyst becomes deactivated relatively rapidly when there is no meals removal or catalyst contacting procedure applied to metals-contaminated heavy oil feed prior to hydrodesulfirization of the heavy oil passing through the fixed bed. It was also determined that using a catalyst bed which comprises a macro-porous catalyst to hydrotreat the heavy oil, prior to passing the heavy oil through the high active hydrodesulfurization catalyst bed, results in a surprisingly high degree of sulfur removal over extended periods of time even though using an equal amount or even less total catalyst than when only the highly active hydrodesulfurization catalyst is used.
However, even the macro-porous catalyst tends to become plugged fairly rapidly if it has moderate or substantial desulfurization activity so that the process becomes economically unattractive because of power loss caused by pressure drop and othe operating difficulties. Nevertheless, inversely grading the catalyst system according to particle size, as disclosed in U.S. Pat. No. 3,496,099, copes with this problem.
U.S. Pat. No. 3,775,290, issued Nov. 27, 1973, discloses a process for hydrotreating a whole desalted crude oil, mixed with a recycled stream from a catalytic cracking unit, such as heavy catalytic cycle oil, before fractionating to produce a gas oil for catalytic cracking.
U.S. Pat. No. 3,891,538, issued June 24, 1975, describes an integrated hydrocarbon conversion process which includes hydrodesulfurizing a heavy hydrocarbon feedstock boiling above 650.degree. F. which is mixed with a cycle oil fraction from a catalytic zone (boiling at about 430.degree.-800.degree. F.) and with a coker gas oil (boiling at about 400.degree.-900.degree. F.) to produce a hydrodesulfurized mixture which, upon fractionating, yields a fraction boiling in the range of 650.degree.-1000.degree. F. for catalytic cracking and a fraction boiling at above 1000.degree. F. for coking.
U.S. Pat. No. 3,893,911, issued July 8, 1975, discloses a process for demetalization of certain petroleum residua and particularly for vanadium removal by initially depositing vanadium on the catalyst during initial hydrogenation contact with metal-containing feedstocks in an ebulliated bed reaction zone, using activated alumina or activated bauxite catalysts, and, if desired, a second stage reaction zone for desulfurization, using a high activity desulfurization catalyst material, such as cobalt, molybdenum, nickel, or oxide and sulfide thereof and the mixtures thereof on a carrier such as alumina, silica, and mixtures thereof.
U.S. Pat. No. 3,976,559, issued Aug. 24, 1976, teaches a process for the combined hydrodesulfurization and hydroconversion of certain heavy asphaltene-containing hydrocarbon feedstocks, such as residua feedstocks. Such hydrocarbon feedstocks are initially contacted with a hydrodesulfurization catalyst which is effective for the selective hydrodesulfurization of the lower-boiling components thereof, thus avoiding conversion of the asphaltene components thereof, while removing between about 30 and 80 percent of the sulfur therein. Subsequently, the partially desulfurized products of this catalytic hydrodesulfurization step are then contacted with an alkali metal in a conversion zone at elevated temperatures in the presence of added hydrogen, so that at least about 90 percent of the sulfur originally contained in the initial hydrocarbon feedstocks is removed therefrom while at least about 50 percent of the 1050.degree. F.+ portion of the feedstock is converted to lower-boiling products. The pore diameter of the catalyst is about 10-100 Angstroms, preferably 20-80 Angstroms, and most preferably 30-50 Angstroms, whereby the asphaltene agglomerates, including most of the metal-containing components, do not have access to the catalyst surfaces thereof, thus avoiding the problems of contamination and deactivation of the catalyst surfaces with these components while accomplishing hydrodesulfurization of lower-boiling components so that 50-80 percent of initially contained suflur in these feedstocks is removed.